Trying to Create a File in Android: Open Failed: Erofs (Read-Only File System)

Trying to create a file in Android: open failed: EROFS (Read-only file system)

I have tried this with and without the WRITE_INTERNAL_STORAGE permission.

There is no WRITE_INTERNAL_STORAGE permission in Android.

How do I create this file for writing?

You don't, except perhaps on a rooted device, if your app is running with superuser privileges. You are trying to write to the root of internal storage, which apps do not have access to.

Please use the version of the FileOutputStream constructor that takes a File object. Create that File object based off of some location that you can write to, such as:

• getFilesDir() (called on your Activity or other Context)
• getExternalFilesDir() (called on your Activity or other Context)

The latter will require WRITE_EXTERNAL_STORAGE as a permission.

Is there an easier way than writing it to a file then reading from it again?

You can temporarily put it in a static data member.

because many people don't have SD card slots

"SD card slots" are irrelevant, by and large. 99% of Android device users will have external storage -- the exception will be 4+ year old devices where the user removed their SD card. Devices manufactured since mid-2010 have external storage as part of on-board flash, not as removable media.

Android Studio open failed: EROFS (Read-only file system) when creating a File

FileOutputStream fos = new FileOutputStream("tours");

This is not pointing anywhere that you can read or write. Always use a method to get at the base locations where you can read and write.

If you are looking to write to internal storage, change this to be something like:

FileOutputStream fos = new FileOutputStream(new File(getFilesDir(), "tours"));

If you are looking to write to external storage, change this to be something like:

FileOutputStream fos = new FileOutputStream(new File(getExternalFilesDir(null), "tours"));

Exception open failed: EROFS (Read-only file system) thrown by

One scenario that can explain this is a change in android:sharedUserId.

android:sharedUserId is an attribute you can put on <manifest>. If 2+ apps have the same sharedUserId and are signed by the same signing key, they can read/write each others files on internal storage.

However, if you change the sharedUserId value for a shipping app — including adding one after having shipped originally without it — existing users will be locked out of their internal storage files. Android switches the Linux uid that the app runs as, but it does not change the ownership of the files. As a result, the app no longer has rights to its files. This is the exception that I would expect to see in this case.

It is this sort of problem that caused Google to finally deprecate android:sharedUserId in Android Q, with a plan to formally discontinue it in a future release.

In your case, I am guessing that you did not set or change android:sharedUserId.

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That is the only scenario that I know of where internal storage for your app would be reported as read-only, where you as the developer caused the problem.

Everything else that I can think of is outside of your control:

• Perhaps the user rooted their device and accidentally changed file ownership or file permissions on your files

• Perhaps the user is a script kiddie who is reverse-engineering your app and is fussing around with android:sharedUserId

• Perhaps the user has been hit with ransomware, and part of its attack was to change permissions on app files via some security flaw

java.io.FileNotFoundException: open failed: EROFS (Read-only file system)

... saves to an external directory

This is not quite right. You are trying to save it in the root system partition which is always read-only.

From javadoc of Environment.getRootDirectory():

Return root of the "system" partition holding the core Android OS. Always present and mounted read-only.

Solution : just save your file somewhere else :

StreamResult result = new StreamResult(new File(android.os.Environment.getExternalStorageDirectory(), "upload_data.xml"));

Note that :

• you must ask the permission to write to the external storage :

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

• External storage is not always available. The partition may not be mounted -yet- when you try to write on it. (just be careful)

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